Fix Slideshow layout shift affecting Chat during image transitions

Use CSS grid stacking instead of absolute positioning so both
entering and leaving images occupy the same grid cell, keeping the
container height stable during crossfade transitions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nginx/vue/src/components/util/Slideshow.vue

@@ -49,33 +49,27 @@ onUnmounted(() => {
 
 <style scoped>
 .slideshow-wrapper {
-    position: relative;
+    display: grid;
     width: 100%;
-    height: 100%;
+    overflow: hidden;
 }
 
 .image-viewer {
+    grid-area: 1 / 1;
     width: 100%;
-    height: 100%;
     overflow: hidden;
 }
 
 img {
     width: 100%;
-    height: 100%;
     object-fit: cover;
+    display: block;
 }
 
 .fade-enter-active,
 .fade-leave-active {
     transition: opacity 0.5s ease;
 }
-.fade-leave-active {
-    position: absolute;
-    top: 0;
-    left: 0;
-    right: 0;
-}
 .fade-enter-from,
 .fade-leave-to {
     opacity: 0;